The present invention relates to a plunger type master cylinder used in a vehicle hydraulic brake system, and particularly a master cylinder which ensures smooth inline suction of brake fluid during automatic brake control and which suppresses invalid stroke of the brake pedal at an initial stage.
As used herein, a plunger type master cylinder refers to a master cylinder including a piston which is in sliding contact with the inner peripheral surfaces of cups held on a cylinder body to pressurize brake fluid in a pressure chamber defined in the cylinder body.
Many modern vehicle hydraulic brake systems have various sophisticated functions including automatic brake control functions, which are initiated by electronic control units while the brake pedal is not being depressed, such automatic brake control functions including traction control (TRC) and electronic vehicle stability control (ESC).
Some of the vehicle hydraulic brake systems having an automatic brake control function are designed so as to suck brake fluid required during automatic braking from a reservoir through its master cylinder. Conventional master cylinders used in this type of vehicle hydraulic brake systems are disclosed in JP patent publications 11-512681 (Publication 1), 2000-108878 (Publication 2) and 2003-170824 (Publication 3).
In Publication 1, the piston of the master cylinder is formed with circumferentially elongated piston ports through which the pressure chamber communicates with the reservoir while the brake pedal is not being depressed. Since the piston ports are circumferentially elongated, its sectional area (passage area) is large, so that when required by hydraulic units, brake fluid can be smoothly supplied through the piston ports. But since the piston ports have a large sectional area, when the brake pedal is depressed, it tends to be pushed in with an extremely light force until the piston ports are completely closed by the primary cup. This will give the driver an uncomfortable feeling as if the brakes are not working at all.
Publication 1 is completely silent about this problem. One solution to this problem would be to reduce the sectional area of the piston ports. But this solution would impair smooth suction of brake fluid through the piston ports.
In Publication 2, the piston is formed with piston ports (relief ports) each having one end thereof open to the outer periphery of the piston and the other end open to the pressure chamber. The piston is further formed with a groove on its outer periphery through which the one end of each piston port is open to the outer periphery of the piston. The groove has a control tapered surface provided rearwardly of the piston ports in order to increase the surface pressure at the contact surfaces of the cup and the piston, and therefore, the groove has a width greater than the diameter of the piston ports. In this arrangement, the piston ports are supposed to be closed by the cup when the rear ends of the piston ports align with the rear end of the radially inner portion of the cup. But actually, since the moderately sloping tapered surface of the groove is provided rearwardly of the piston ports, the piston ports tend to be closed when the rear ends of the radially inner portion of the cup reaches an indeterminate point of the tapered surface. Thus, the position of the piston where the piston ports are closed by the cup cannot be determined accurately.
Moreover, because the depth of the groove having the control tapered surface is limited for sufficient interference of the cup, and because the radially inner portion of the cup tends to be pulled into the groove while the piston is not being operated, the effective sectional area of the passage through which brake fluid flows scarcely increases.
Publication 3 discloses a plunger type master cylinder having a piston formed with a plurality of piston ports and an annular groove through which the piston ports communicate with the reservoir. In this arrangement, since the annular groove has a width smaller than the diameter of the piston ports, brake fluid cannot sufficiently smoothly flow through the annular groove. Thus, if this master cylinder is used in a hydraulic brake system in which high suction rate is required during automatic braking, brake fluid cannot be supplied at a sufficiently high rate. This deteriorates brake response.
Another conventional master cylinder includes a valve mechanism for reducing the sectional area of the piston ports or any other passage through which the pressure chamber communicates with the reservoir when the brake pedal is depressed. Such a valve mechanism is however not preferable because it will complicate the structure and push up the cost.
An object of the invention is to provide a master cylinder of the above type which ensures smooth suction of brake fluid during automatic brake control and which suppresses an initial invalid stroke of the brake pedal.